| With people’s increased awareness of protecting the environment, the stricter requirements of environment protection law, and the intrinsic requirement for building a beautiful China, the development trend of diesel is the ultra clean. Under the situation of crude oil becoming poorer and heavier in quality, existing equipment and technology condition has been gradually unable to meet the production requirements of more high quality diesel. Therefore, it is necessary to enhance the catalytic performance of desulfurization catalyst. The paper based on the big loss ratio of the intial catalytic activity for diesel hydrodesulfurizaiton over Mo-Co/γ-Al2O3 catalyst which was produced by FRIPP, the influence factors on catalytic acitivity, reaction and deactivation of the hydrodesulfurizaiton were investigated on the lab scale. The influence factors of the hydrogenation desulfurization reaction and the deactivation mechanism of the catalyst were investigated. It’s pave the way to provide theoretical support for further improving the desulfurization performance of the catalyst.Firstly, the effect of sulfur content in stock, reaction temperature, liquid hourly space velocity (LHSV) and the volume ratio of hydrogen to oil on catalyst activity was studied on a fixed bed. The results showed that the desulfurization rate of diesel over Mo-Co/γ-Al2O3 catalyst kept stable after the rapid deactivation in the early stage of 140 minutes, the loss ratio of HDS activity was nearly 1/3. The reaction conditions were optimized as follows:the temperature of 340℃, LHSV of 1.4h-1 to 1.8h-1, volume ratio of hydrogen to oil of 300. Under the optimal conditions the initial HDS rate reached up to 97%, and kept stable at 78.6% after the reation time of 140 mins. The loss ratio of HDS activity was nearly 20%.Secondly, the comparative characterization of fresh, sulfide and used catalysts was carried out. The results showed that there is no loss in content of Mo and only 3.85% loss in content of Co for the used Mo-Co/γ-Al2O3 catalyst compared with the fresh one. And the specific surface area and total pore volume of the used one lost 30% and 33%. That means that the catalytic activity decline in the early stage can be attributed to the carbon deposition. The deposited carbon was characterized by IR and GC-MS, the components of carbon deposition mainly consisted of of long chain alkanes, about C14-C25 and 2-3 benzene rings of alkyl aromatics. The used catalyst was regenerated by roasting at the temperature of 500℃ for one hour, and the acitivity of HDS for the regenerated catalyst can reach up to 94%.Furthermore, in-situ FT-IR technology was employed to investigate the surface reaction process for the diesel HDS by thiophene as the model compounds. The results showed that the species of Co, Mo were difficult to be vulcanized at low temperature, the curing reaction began at 320℃. During the thiophene adsorption and desorption proess over the catalyst, thiophene can be adsorbed on the support of γ-Al2O3 and active metal. The former can be attributed to the physical adsorption, and it’s very easy to desorption by increasing the temperature. The peak of 1425 cm-1 in IR spectrum showed that thiophene was adsorbed on the species of unsaturated Mo to form the bond of S-M, the electron was transferred from the bond of C-S to S-M, which can destroy the aromaticity of thiophene and weaken the C-S bond to achieve the purpose of desulfurization. On the reaction process at high temperature with the atomosphere of hydrogen, the characteristic peaks of 2952cm-1 and 2883cm-1 indicate that the bond C-S key was cracked, and the S was removed in the form of H2S, and the purpose of desulfurization was obtained by thiophene changing to C4 species.Finally, the mechanism for the coke deposition over the catalyst was also discussed. The carbocations were formed from the sulfocompound which was desorptive S atom during hydrodesulfurization reaction, then to form the coke deposition undergo the isomerization and alkylation over the acid sites in catalyst. |
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